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  • Scala: Matching optional Regular Expression groups

    - by Brian Heylin
    I'm trying to match on an option group in Scala 2.8 (beta 1) with the following code: import scala.xml._ val StatementPattern = """([\w\.]+)\s*:\s*([+-])?(\d+)""".r def buildProperty(input: String): Node = input match { case StatementPattern(name, value) => <propertyWithoutSign /> case StatementPattern(name, sign, value) => <propertyWithSign /> } val withSign = "property.name: +10" val withoutSign = "property.name: 10" buildProperty(withSign) // <propertyWithSign></propertyWithSign> buildProperty(withoutSign) // <propertyWithSign></propertyWithSign> But this is not working. What is the correct way to match optional regex groups?

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  • Why is Scala very complex?

    - by Anantha Kumaran
    I am a student. I learned java during the 2nd year. Now i am in fourth year. I got bored with java and i started to learn Scala. As i learn it, i found it being very complex (although i love it). My question may apply to all new complex language. Why scala is complex? is it because we need to create complex softwares? or i am the only one who thinks it is complex?

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  • Scala downwards or decreasing for loop?

    - by Felix
    In scala, you often use an iterator to do a for loop in an increasing order like: for(i <- 1 to 10){ code } How would you do it so it goes from 10 to 1? I guess 10 to 1 gives an empty iterator (like usual range mathematics)? I made a scala script which solves it by calling reverse on the iterator, but it's not nice in my opinion, is this the way to go: def nBeers(n:Int) = n match { case 0 => ("No more bottles of beer on the wall, no more bottles of beer."+ "\nGo to the store and buy some more, "+ "99 bottles of beer on the wall.\n") case _ => (n+" bottles of beer on the wall, "+n +" bottles of beer.\n"+"Take one down and pass it around, "+ (if((n-1)==0) "no more" else (n-1))+ " bottles of beer on the wall.\n") } for(b <- (0 to 99).reverse)println(nBeers(b)) ?? Any comments/suggestions?

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  • How do i change the Scala version that sbt works with?

    - by ashy_32bit
    Firing up the SBT console it reads : [info] Building project AYLIEN 1.0 against Scala 2.8.1 [info] using MyProject with sbt 0.7.4 and Scala 2.7.7 How can I make it use MyProject with sbt 0.7.4 and Scala 2.8.1 ? Please pay attenetion that I'm not asking about the Scala version that is used to build my project (it is the 2.8.1 as you can see), but I rather want to make sbt use MyProject with Scala 2.8.1. Apparently sbt uses it's own scala version to work with project definition (MyProject here) which is different than one it uses to actually build the project! or perhaps I'm missing something ... ?

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  • Scala: "Parameter type in structural refinement may not refer to an abstract type defined outside th

    - by raichoo
    Hi, I'm having a problem with scala generics. While the first function I defined here seems to be perfectly ok, the compiler complains about the second definition with: error: Parameter type in structural refinement may not refer to an abstract type defined outside that refinement def >>[B](a: C[B])(implicit m: Monad[C]): C[B] = { ^ What am I doing wrong here? trait Lifter[C[_]] { implicit def liftToMonad[A](c: C[A]) = new { def >>=[B](f: A => C[B])(implicit m: Monad[C]): C[B] = { m >>= (c, f) } def >>[B](a: C[B])(implicit m: Monad[C]): C[B] = { m >> a } } } IMPORTANT: This is NOT a question about Monads, it's a question about scala polymorphism in general. Regards, raichoo

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  • Remove redundant entries, scala way

    - by andersbohn
    Edit: Added the fact, that the list is sorted, and realizing 'duplicate' is misleading, replaced that with 'redundant' in the title. I have a sorted list of entries stating a production value in a given interval. Entries stating the exact same value at a later time adds no information and can safely be left out. case class Entry(minute:Int, production:Double) val entries = List(Entry(0, 100.0), Entry(5, 100.0), Entry(10, 100.0), Entry(20, 120.0), Entry(30, 100.0), Entry(180, 0.0)) Experimenting with the scala 2.8 collection functions, so far I have this working implementation: entries.foldRight(List[Entry]()) { (entry, list) => list match { case head :: tail if (entry.production == head.production) => entry :: tail case head :: tail => entry :: list case List() => entry :: List() } } res0: List[Entry] = List(Entry(0,100.0), Entry(20,120.0), Entry(30,100.0), Entry(180,0.0)) Any comments? Am I missing out on some scala magic?

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  • What are the exact versions of stuff you have to install in order to be able to step-debug a Scala p

    - by Alex R
    How do YOU debug a Scala program? I mean YOU as in the person posting the Answer :) Please answer only from personal experience, not from stuff you've heard or read on the Internet. You should not believe everything you read on the Internet, especially tales of complex open-source software configurations that actually work :-) The are many Java tools which claim to support Scala in some way or another, but I have so far struck out in trying to get any one of them to actually let me set a breakpoint in Scala code and step through it. These are big, major open-source IDEs I'm talking about here. The main problem in getting a debugger to work seems to be the "version hell" with fast-changing IDEs, Plug-Ins, JDKs, and the Scala language itself. Hence, the more detailed re-statement of the question is appropriate: What is the exact version number of the IDE, Plug-In, JDK, Scala, and even Operating System, that you are successfully using? My question is related to this one, but wider in scope: http://stackoverflow.com/questions/2272705/how-to-debug-scala-code-when-outside-of-an-ide Thanks

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  • make a lazy var in scala

    - by ayvango
    Scala does not permit to create laze vars, only lazy vals. It make sense. But I've bumped on use case, where I'd like to have similar capability. I need a lazy variable holder. It may be assigned a value that should be calculated by time-consuming algorithm. But it may be later reassigned to another value and I'd like not to call first value calculation at all. Example assuming there is some magic var definition lazy var value : Int = _ val calc1 : () => Int = ... // some calculation val calc2 : () => Int = ... // other calculation value = calc1 value = calc2 val result : Int = value + 1 This piece of code should only call calc2(), not calc1 I have an idea how I can write this container with implicit conversions and and special container class. I'm curios if is there any embedded scala feature that doesn't require me write unnecessary code

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  • scala xml rewrite rule (or, simple pattern help)

    - by williamstw
    I'm missing some fairly simple syntax I gather. I'm trying to rewrite an element label to something else and keep everything else intact. object htmlRule extends RewriteRule { override def transform(n: Node): Seq[Node] = n match { case Elem(prefix, "document", attribs, scope, child@_*) => Elem(prefix, "html", attribs, scope, child) case other => other } } Now, I ask for an explanation of two things: 1) What exactly does "child@_*" mean in plain English? 2) How can I capture the value of "child@_*" and just let it pass right through to the new element? Currently, I get the following error, which makes sense. [error] found : Seq[scala.xml.Node] [error] required: scala.xml.Node [error] Elem(prefix, "html", attribs, scope, child) I'm not wedded to this either, so if there's a better way to simply change the element name of a specific node, let's here it... Thanks, --tim

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  • Remove duplicates in entries, scala way

    - by andersbohn
    I have a list of entries stating a production value in a given interval. Entries stating the exact same value at a later time adds no information and can safely be left out. case class Entry(minute:Int, production:Double) val entries = List(Entry(0, 100.0), Entry(5, 100.0), Entry(10, 100.0), Entry(20, 120.0), Entry(30, 100.0), Entry(180, 0.0)) Experimenting with the scala 2.8 collection functions, so far I have this working implementation: entries.foldRight(List[Entry]()) { (entry, list) => list match { case head :: tail if (entry.production == head.production) => entry :: tail case head :: tail => entry :: list case List() => entry :: List() } } res0: List[Entry] = List(Entry(0,100.0), Entry(20,120.0), Entry(30,100.0), Entry(180,0.0)) Any comments? Am I missing out on some scala magic?

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  • what good orm api will work well with scala or erlang

    - by Emotu Balogun
    I'm considering taking up scala programming but i'm really concerned about what will become of my ORM based applications. I currently use hibernate as my ORM and i find it a really reliable tool. I'd like to know if there's any ORM tool as efficient but written in scala, or will hibernate work seamlessly with it. i don't want to have to start writing endless sql queries again (like the days of JDBC). I also have the same thought about erlang. is there a good orm out there for erlang?? and can i use erlang with other DBMS like oracle and mysql with ORM

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  • Generic object load function for scala

    - by Isaac Oates
    I'm starting on a Scala application which uses Hibernate (JPA) on the back end. In order to load an object, I use this line of code: val addr = s.load(classOf[Address], addr_id).asInstanceOf[Address]; Needless to say, that's a little painful. I wrote a helper class which looks like this: import org.hibernate.Session class DataLoader(s: Session) { def loadAddress(id: Long): Address = { return s.load(classOf[Address], id).asInstanceOf[Address]; } ... } So, now I can do this: val dl = new DataLoader(s) val addr = dl loadAddress(addr_id) Here's the question: How do I write a generic parametrized method which can load any object using this same pattern? i.e val addr = dl load[Address](addr_id) (or something along those lines.) I'm new to Scala so please forgive anything here that's especially hideous.

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  • What are the exact versions of stuff you had to install in order to be able to step-debug a Scala pr

    - by Alex R
    How do YOU debug a Scala program? I mean YOU as in the person posting the Answer :) Please answer only from personal experience, not from stuff you've heard or read on the Internet. You should not believe everything you read on the Internet, especially tales of complex open-source software configurations that actually work :-) The are many Java tools which claim to support Scala in some way or another, but I have so far struck out in trying to get any one of them to actually let me set a breakpoint in Scala code and step through it. These are big, major open-source IDEs I'm talking about here. The main problem in getting a debugger to work seems to be the "version hell" with fast-changing IDEs, Plug-Ins, JDKs, and the Scala language itself. Hence the second part of my question, which is really the most important part: What is the exact version number of the IDE, Plug-In, JDK, Scala, and even Operating System, that you are successfully using? My question is related to this one, but wider in scope: http://stackoverflow.com/questions/2272705/how-to-debug-scala-code-when-outside-of-an-ide Thanks

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  • Can I reproduce Scala's behavior for == ?

    - by JPP
    In Programming in Scala, I can read that the == operator behaves as if it was defined like this: final def == (that: Any): Boolean = if (null eq this) {null eq that} else {this equals that} But there must actually be compiler magic to avoid null pointer exceptions, right? Is there any way for me to replicate this behavior with pure Scala; i.e., have an operator/method return one thing if the receiver is null and another one if it isn't? What I mean is an actual implementation of null eq this. I suppose I can write a "pimp" and then define the method on the wrapper class, but is there a more direct way to do this?

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  • Merging elements in a scala list

    - by scompt.com
    I'm trying to port the following Java snippet to Scala. It takes a list of MyColor objects and merges all of the ones that are within a delta of each other. It seems like a problem that could be solved elegantly using some of Scala's functional bits. Any tips? List<MyColor> mergedColors = ...; MyColor lastColor = null; for(Color aColor : lotsOfColors) { if(lastColor != null) { if(lastColor.diff(aColor) < delta) { lastColor.merge(aColor); continue; } } lastColor = aColor; mergedColors.add(aColor); }

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  • Logging in over and over again. How to fix this?

    - by romeovs
    Ok, I messed up. I installed ubuntu 11.10, installed awesome wm and removed unity, to have something to fall back on, I also installed gnome-session-fallback. I was messing around and did the following, because the awesome wiki told me to: gconftool-2 --type bool --set /apps/nautilus/preferences/show_desktop False # Still disable the buggy Nautilus desktop thing gconftool-2 --type string --set /desktop/gnome/session/required_components/windowmanager awesome # sets awesome as wm Now here's what's wrong: I can start up decently, and then I get into a login window (that of gnome-session-fallback). I enter my username, select the preferred window manager (awesome in my case) and enter my password. It accepts these, but then hold for a second and just opens the login window again, in effect preventing me from actually logging in. I also tried gconftool-2 --unset (from the tty) on these settings, but that didn't work either. What can I do to revert the gconftool-2 settings to something that should work? I tried apt-get purging gnome-session-fallback and lightdm, and then installing them again, but that didn't work.

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  • How to change attribute on Scala XML Element

    - by Dave
    I have an XML file that I would like to map some attributes of in with a script. For example: <a> <b attr1 = "100" attr2 = "50"> </a> might have attributes scaled by a factor of two: <a> <b attr1 = "200" attr2 = "100"> </a> This page has a suggestion for adding attributes but doesn't detail a way to map a current attribute with a function (this way would make that very hard): http://www.scalaclass.com/book/export/html/1 What I've come up with is to manually create the XML (non-scala) linked-list... something like: // a typical match case for running thru XML elements: case Elem(prefix, e, attributes, scope, children @ _*) => { var newAttribs = attributes for(attr <- newAttribs) attr.key match { case "attr1" => newAttribs = attribs.append(new UnprefixedAttribute("attr1", (attr.value.head.text.toFloat * 2.0f).toString, attr.next)) case "attr2" => newAttribs = attribs.append(new UnprefixedAttribute("attr2", (attr.value.head.text.toFloat * 2.0f).toString, attr.next)) case _ => } Elem(prefix, e, newAttribs, scope, updateSubNode(children) : _*) // set new attribs and process the child elements } Its hideous, wordy, and needlessly re-orders the attributes in the output, which is bad for my current project due to some bad client code. Is there a scala-esque way to do this?

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  • How are Scala closures transformed to Java objects?

    - by iguana
    I'm currently looking at closure implementations in different languages. When it comes to Scala, however, I'm unable to find any documentation on how a closure is mapped to Java objects. It is well documented that Scala functions are mapped to FunctionN objects. I assume that the reference to the free variable of the closure must be stored somewhere in that function object (as it is done in C++0x, e.g.). I also tried compiling the following with scalac and then decompiling the class files with JD: object ClosureExample extends Application { def addN(n: Int) = (a: Int) => a + n var add5 = addN(5) println(add5(20)) } In the decompiled sources, I see an anonymous subtype of Function1, which ought to be my closure. But the apply() method is empty, and the anonymous class has no fields (which could potentially store the closure variables). I suppose the decompiler didn't manage to get the interesting part out of the class files... Now to the questions: Do you know how the transformation is done exactly? Do you know where it is documented? Do you have another idea how I could solve the mystery?

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  • Scala implicit dynamic casting

    - by weakwire
    I whould like to create a scala Views helper for Android Using this combination of trait and class class ScalaView(view: View) { def onClick(action: View => Any) = view.setOnClickListener(new OnClickListener { def onClick(view: View) { action(view) } }) } trait ScalaViewTrait { implicit def view2ScalaView(view: View) = new ScalaView(view) } I'm able to write onClick() like so class MainActivity extends Activity with ScalaViewTrait { //.... val textView = new TextView(this) textView.onClick(v => v.asInstanceOf[TextView] setText ("asdas")) } My concern is that i want to avoid casting v to TextView v will always be TextView if is applied to a TextView LinearLayout if applied to LinearLayout and so on. Is there any way that v gets dynamic casted to whatever view is applied? Just started with Scala and i need your help with this. UPDATE With the help of generics the above get's like this class ScalaView[T](view: View) { def onClick(action: T => Any) = view.setOnClickListener(new OnClickListener { def onClick(view: View) { action(view.asInstanceOf[T]) } }) } trait ScalaViewTrait { implicit def view2ScalaView[T](view: View) = new ScalaView[T](view) } i can write onClick like this view2ScalaView[TextView](textView) .onClick(v => v.setText("asdas")) but is obvious that i don't have any help from explicit and i moved the problem instead or removing it

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  • Scala XML API: Why allow NodeSeq as attribute values?

    - by Synesso
    It seems attribute values are of type Seq[Node]. scala> <a b="1"/>.attribute("b") res11: Option[Seq[scala.xml.Node]] = Some(1) This means you can assign XML as an attribute value. scala> <a b={<z><x/></z>}/>.attribute("b") res16: Option[Seq[scala.xml.Node]] = Some(<z><x></x></z>) scala> <a b={<z><x/></z>}/>.attribute("b").map(_ \ "x") res17: Option[scala.xml.NodeSeq] = Some(<x></x>) scala> new xml.PrettyPrinter(120, 2).format(<a b={<z><x/></z>}/>) res19: String = <a b="<z><x></x></z>"></a> This seems funky to me. I've never seen XML as attribute values in the real world. Why is it allowed? Why is an attribute value simply not of type String?

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  • scala 2.8 CanBuildFrom

    - by oxbow_lakes
    Following on from another question I asked, I wanted to understand a bit more about the Scala method TraversableLike[A].map whose signature is as follows: def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Repr, B, That]): That Notice a few things about this method: it takes a function turning each A in the traversable into a B it returns That and takes an implicit argument of type CanBuildFrom[Repr, B, That] I can call this as follows: > val s: Set[Int] = List("Paris", "London").map(_.length) s: Set[Int] Set(5,6) What I cannot quite grasp is how the fact that That is bound to B (i.e. it is some collection of B's) is being enforced by the compiler. The type parameters look to be independent in both the signature above and in the signature of the trait CanBuildFrom itself: trait CanBuildFrom[-From, -Elem, +To] How is the scala compiler ensuring that That cannot be forced into something which does not make sense? > val s: Set[String] = List("Paris", "London").map(_.length) //will not compile EDIT - this question of course boils down to: How does the compiler decide what implicit CanBuildFrom objects are in scope for the call?

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